Gram-positive microorganisms, such as members of the group Bacillus, have been used for large-scale industrial fermentation due, in part, to their ability to secrete their fermentation products into the culture media. In gram-positive bacteria, secreted proteins are exported across a cell membrane and a cell wall, and then are subsequently released into the external media usually maintaining their native conformation.
Various gram-positive microorganisms are known to secrete extracellular and/or intracellular protease at some stage in their life cycles. Many proteases are produced in large quantities for industrial purposes. A negative aspect of the presence of proteases in gram-positive organisms is their contribution to the overall degradation of secreted heterologous or foreign proteins.
The classification of proteases found in microorganisms is based on their catalytic mechanism which results in four groups: the serine proteases; metallo-proteases; cysteine proteases; and aspartic proteases. These categories, in general, can be distinguished by their sensitivity to various inhibitors. For example, the serine proteases are inhibited by phenylmethylsulfonylfluoride (PMSF) and diisopropylfluorophosphate (DIFP); the metallo-proteases by chelating agents; the cysteine enzymes by iodoacetamide and heavy metals and the aspartic proteases by pepstatin. The serine proteases have alkaline pH optima, the metalloproteases are optimally active around neutrality, and the cysteine and aspartic enzymes have acidic pH optima (Biotechnology Handbooks, Bacillus. vol. 2, edited by Harwood, 1989 Plenum Press, New York).
Metallo-proteases form the most diverse of the catalytic types of proteases. Family M23 contains bacterial enzymes such as the β-lytic endopeptidases of Lysobacter and Achromobacter and the Pseudomonas LasA protein and have specificity for Gly bonds, especially in Gly-Gly+Xaa-sequences (Methods in Enzymology, vol. 248, Academic Press, Inc. 1994). The enzymes of the M23 family contain zinc and a conserved His-Xaa-His motif.